1. Field of the Invention
The invention concerns a detector of electromagnetic waves and, more precisely, a detector of electromagnetic waves based on quantum well semiconductors.
2. Description of the Prior Art
At present, several approaches may be used to detect an electromagnetic wave belonging to the mean infrared range, i.e. one with a wavelength that is situated typically between 2 and 20 .mu.m. Among these approaches, we might cite the use of:
(a) semiconductor materials of the II-IV and IV-VI groups; PA0 (b) doped semiconductor materials of the IV group; PA0 (c) associated III-V semiconductor materials in multiple quantum well structures. PA0 B. F. Levine, K. K. Choi, C. G . Bethea, J. Walker and R. J. Malik, "New 10 .mu.m detector using intersubband absorption in resonant tunneling GaAlAs superlattices", APL 50 (26), 20 April 1987, p. 1092. PA0 K. K. Choi, B. F. Levine, C. G. Bethea, J. Walker and R. J. Malik, "Multiple quantum well 10 .mu.m GaAs/Al.sub.(x) Ga.sub.(1-x))As infrared detector with improved responsivity", APL 50 (25), 22 June 1987, p. 1814. PA0 B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker and R. J. Malik, "Quantum well avalanche multiplication initiated by 10 .mu.m intersubband absorption and photoexcited tunnelling", APL 51 (12), 21 September 1987, p. 934. PA0 B. F. Levine, C. G. Bethea, G. Hasnain, J. Walker, R. J. Malik, "GaAs/AlGaAs quantum-well long-wavelength infrared (LWIR) detector with a detectivity comparable to HgCdTe", Elec. Letters, Vol. 24, No. 12, 9/6/88, p. 747. PA0 B. F. Levine, C. G. Bethea, G. Hasnain, J. Walker, R. J. Malik, "High detectivity D*=1.0.times.10.sup.10 cm.sqroot.Hz/W GaAs/AlGaAs multiquantum well 8.3 .mu.m detector", APL 53(4), 25 July 1988, p 296. PA0 a semiconductor structure having at least one stacking of first, second, third, fourth and fifth layers, the widths of the forbidden gaps of which can be used to obtain the following profile of potential energy corresponding to the bottom of the conduction band for the electrons PA0 relatively low energy for the first layer giving a reference (0) of the energies; PA0 intermediate energy for the second and third layers; PA0 energy of higher value for the fourth and fifth layers; PA0 means for populating the first permitted level (e.sub.1) of energy of the quantum well with electrons; PA0 means for applying an electrical field perpendicularly to the plane of the structure. PA0 a semiconductor structure having at least one stacking of first, second, third, fourth and fifth layers, the forbidden gap widths of which make it possible to obtain the following profile of potential energy corresponding to the top of the valence band for the holes: PA0 relatively low energy for the first layer giving a reference (0) of the energies; PA0 intermediate energy for the second and third layers; PA0 energy of higher value for the fourth and fifth layers; PA0 means for populating the first permitted level of energy of the quantum well with holes.
The first approach (a) consists in using semiconductor materials, the forbidden gap width of which is smaller than the photon energy h.nu. of the wave to be detected which therefore makes it possible to transfer electrons from the valence band to the conduction band. These electrons are then collected by means of an external circuit and are the cause of a photocurrent that enables the detection (see FIG. 1a). Among the materials with a forbidden gap width compatible with this type of operation in the mean infrared range, we might cite the II-VI group alloys of the HgCdTe type and the IV-VI group alloys of the PbSnTe type.
The second approach (b) providing for the use of semiconductors with a larger forbidden gap width than the photon energy to be detected is possible by resorting to a doping of the materials used. This doping reveals an electron donor level corresponding to the impurities that cause the doping. From this energy level, which is closer to the bottom of the conduction band than is the top of the valence band, it will be possible for electron transitions to occur towards the conduction band, freeing the electrons that have undergone these transitions under the effect, in particular, of an infrared electromagnetic field (see FIG. 1b) and making them detectable. For example, the doping of silicon with arsenic (Si:As) or of germanium with mercury (Ge:Hg) enables detection in a special spectral range located, in terms of wavelength, around 10 microns.
The third approach (c) is based on the occurrence of the electron transitions between permitted levels of energy (e.sub.1 and e.sub.2) within the conduction band of semiconductor quantum structures. FIG. 1c gives an example of this type of transition in a well having two discrete levels of energy permitted for the electrons. By the application of an electrical field to this type of configuration, it is possible, as indicated in FIGS. 2a and 2b, to make an extraction from the well, in a preferred way, of the electrons located at the excited quantum level (e2 in FIG. 2a and e'2 in FIG. 2b). Thus, the collection, in the external electrical circuit, of these electrons coming from the second quantum level to which they have been taken by an infrared illumination (h.nu.), enables the detection of this illumination. The approach which is an object of the invention concerns infrared detectors of this third type (c).
The type of detector to which the invention can be applied is therefore based on the occurrence, under the effect of an infrared illumination, of electron transitions within the conduction band of semiconductor quantum wells. The principle of operation of these detectors is therefore based on the use of these transitions to place the electrons initially located at the fundamental level of the wells at a level enabling them to easily leave the well under the effect of an applied electrical field. Up till now, two configurations corresponding to the approaches of the above-described FIGS. 2a and 2b have been proposed to make this type of operation possible. The approach shown in FIG. 2a uses a quantum well displaying two levels e.sub.1 and e.sub.2 of permitted energy for the electrons in the conduction band. Under the effect of an illumination represented by its photon energy hv in FIG. 2a, electron transitions may occur from the level e.sub.1 towards the level e.sub.2. By the application of an electrical field E to the structure, it is then possible to make the electrons located at the level e.sub.2 cross the potential barrier .phi. in order to extract them from the well. The electrons then participate in a photocurrent enabling the detection of the illumination.
Devices of this type have been described in the following articles:
The configuration of FIG. 2b is based on the use of a well having only one bound level e1, separated from the top of the barrier forming the well by an energy close to the photon energy h.nu. of the electromagnetic wave to be detected (see FIG. 2b). The transitions occur between this level and a virtual level located in the continuum.
Devices of this type have been described in the following articles:
To obtain a major absorption of the illumination to be detected, a large number of wells is used within the detectors based on this quantum principle, and this is done irrespectively of the approach chosen, whether that of FIG. 2a or of FIG. 2b. Thus, the conduction band of these multiple quantum well (MQW) structures can be symbolized by the FIGS. 3a and 3b. These FIGS. 3a and 3b respectively correspond to two types of transitions that have just been evoked with respect to the FIGS. 2a and 2b. This type of structure is based on the periodic stacking of layers of a first material M.sub.1 and a second material M.sub.2. The widths of the forbidden gaps of the two materials are different in order to obtain potential wells for the electrons in the conduction band The contact layers located on both sides of the periodic region designated by the MQW may be formed, for example, by layers of n doped material M.sub.1 referenced M.sub.1 n+. To increase the number of possible transitions, generally n type doping of the MQW zone is resorted to.
Among the characteristics shown by these two approaches, the following elements may be noted, in referring again to FIGS. 2a and 2b:
in the case of the approach of FIG. 2a, the transitions occur between two levels located beneath the potential barrier between the wells. Consequently, the electrons located at the two levels have high confinement in the width 2d.sub.1 of the wells, whence a high transition efficiency between the two levels. The working of this type of detector takes advantage of the difference in mobility in the direction perpendicular to the plane of the layers forming the wells, between the electrons located on the levele e.sub.1 and those located on the level e.sub.2. For example, to be extracted from the well, the electrons located at the level e.sub.2 have a lower potential barrier .phi. to cross than do the electrons located at the level e.sub.1 retained in a field in the well by a barrier .phi. +h.nu.). Furthermore, the width of this barrier is smaller (l.sub.2) for the level e.sub.2 than for the level e.sub.1 (l.sub.1). This is so because of the potential profile communicated to the structure by the electrical field used for the collection of the photoelectrons. Nevertheless, this potential barrier to be crossed restricts the performance characteristics of the detector because it limits the difference in mobility in the transport perpendicular to the semiconductor layers between the electrons located at the level e.sub.1 and those located at the level e.sub.2, the two levels being separated by the energy h.nu. given by the central wavelength of the detector response curve.
If we now look at FIG. 2b, it can be seen, on this assumption, that the electrons excited by the electromagnetic field to be detected no longer have any barrier to cross in order to be extracted from the well, and this favors their participation in the photocurrent. On the contrary, the level 3'.sub.2 to which the photo-excited electrons are taken from the level e.sub.1 is virtual, and the probability of transition is far smaller than in the case of FIG. 2a, given the very low level of localization, in the zone of the well, of the electrons located at this virtual level e'2. In other words, the wave function associated with this level is not confined to the zone of the well.
However, as a comparison between the two known configurations (FIGS. 2a and 2b), it may also be pointed out that in the first example (FIG. 2a), the distance l.sub.2 depends directly on the field applied to the structure. The more it is sought to reduce l.sub.2, the more does it become necessary to apply a strong field and the more is l.sub.1 reduced. This has the consequence of increasing the dark current of the structure. The problem is different in the case of FIG. 2b where weaker fields may be enough for the efficient collection of the photo-excited carriers but where, as we have seen, there is the problem of the low probability of transition between the fundamental level of the well and the excited virtual level. The known approaches, therefore, do not provide for the association of a high-level response, due to a high probability of transition between a fundamental level and an excited level e'2, on the one hand, and a very low dark current on the other. This becomes possible through the invention proposed herein, as can be seen in the following paragraph.
To provide an answer to the problem of obtaining a high response and a low dark current on one and the same detector based on the occurrence of interband transitions within the conduction band of multiple quantum well structures, the barrier to be crossed should be very different for the electrons located on the excited level and for those located on the fundamental level. The solution proposed consists in giving the well a particular potential profile.